Dave's Garden Website has lots of information about Conophytum here; this species is only one of many, many different and wonderful types: Conophytum obcordellum.

Hope that helps narrow it down a bit - you've got your work cut out to try and figure out exactly what they are - as these hybridize easily, if this one is seed grown, you will never be able to pin it down exactly, but at least that gives you some guidelines to grow them successfully (although, judging by the pictures, benign neglect suits them just fine!)